Pelargonium capitatum flowers

In this cluster of Pelargonium capitatum flowers the anthers at the tips of the pale filaments represent different stages of development. The orange ones are ripe, ahead of the purple ones. An anther will only open to release pollen once it is ripe.

Viable seed depends on a pollinating agent seeking nectar carrying ripe pollen to a receptive stigma on another flower. Ripe pollen is sticky for attaching to the body of the pollinator (often a bird or insect).

Pollinators will visit several flowers, each yielding only a small enough, sweet enough sip of nectar to motivate the traveller for a number of visits. Visits to flowers of the same species allow for successful pollination. By the time the pollinator is sated, some pollen has usually been delivered to several destinations, some hopefully to "the right" addresses. 

Every surviving feature in an evolutionary design is aligned with the probability of success. For viable pollen to reach the stigma of another flower of its species, maximal flower production over the longest possible season must happen in places where suitable pollinators can be fed to multiply sufficiently for continuing the service (Wikipedia).